A hard-disk drive (HDD) is a non-volatile storage device that is housed in a protective enclosure and stores digitally encoded data on one or more circular disks having magnetic surfaces (a disk may also be referred to as a platter). When an HDD is in operation, each magnetic-recording disk is rapidly rotated by a spindle system. Data is read from and written to a magnetic-recording disk using a read/write head which is positioned over a specific location of a disk by an actuator.
A read/write head uses a magnetic field to read data from and write data to the surface of a magnetic-recording disk. As a magnetic dipole field decreases rapidly with distance from a magnetic pole, the distance between a read/write head and the surface of a magnetic-recording disk must be tightly controlled. To provide a uniform distance between a read/write head and the surface of a magnetic-recording disk, an actuator relies on air pressure inside the hard drive enclosure to support the read/write heads at the proper distance away from the surface of the magnetic-recording disk while the magnetic-recording disk rotates. A read/write head therefore is said to “fly” over the surface of the magnetic-recording disk. That is, the air pulled along by a spinning magnetic-recording disk forces the head away from the surface of the magnetic-recording disk. When the magnetic-recording disk stops spinning, a read/write head must either “land” or be pulled away.
A write-head of an HDD records data onto the surface of a magnetic-recording disk in a series of concentric tracks. The greater the frequency of the magnetic write field used to write data to a track on the magnetic-recording disk, the greater the amount of data that can be stored on the track (referred to as recording density).
While it is desirable to store as much data as possible on a magnetic-recording disk, there are practical limits on how much data can be recorded to a single track. The chance of encountering an error in reading data written to a track increases as the density of the data written to the track increases. Also, the velocity of the disk increases with distance from the center of the disk; thus, the density of data written to a disk tends to be greater near the center of the disk because tracks near the center of the disk move slower under the write-head than tracks further away from the center of the disk. As a result, the frequency at which data is written (the “write frequency”) to a particular track of a platter is typically selected to be the highest frequency at which data may be safely read from the track nearest the center of the disk without exceeding a particular error rate.
Some hard-disk drives may use an approach referred to as zone bit recording. In zone bit recording, a magnetic-recording disk is logically divided into a plurality of concentric zones. Each of the plurality of zones may include a number of different tracks on the magnetic-recording disk. For example, a hard-disk drive may logically divide the disk into a set of 10 concentric zones, and each concentric zone may include 10% of the tracks on the magnetic-recording disk. The hard-disk drive writes data to tracks within the same concentric zone at the same write frequency; however, the hard-disk drive writes data to tracks of different concentric zones at different write frequencies. The hard-disk drive may increase the write frequency when moving from one concentric zone to an adjacent concentric zone that is further from the center of the disk, and similarly decrease the write frequency when moving from one concentric zone to an adjacent concentric zone that is closer to the center of the disk. In this way, the change in the write frequency between different concentric zones may help offset the natural tendency for the density of data to decrease with distance from the center of the disk due to the increase in speed at which the tracks move under the write-head. As a result, the recording density across the plurality zones on the disk is kept within a desired range to ensure that the soft error rate does not exceed an undesirable level.